An estimated 270,000 of the latest model of Prius could be withdrawn. Photograph: Everett Kennedy Brown/EPA

Toyota's reputation was in danger of plunging into freefall tonight when it admitted investigating reports of brake faults in Prius hybrid cars in the US and Japan in a move that could trigger another recall.

The possibility that an estimated 270,000 of the latest model of the ­Japanese company's flagship vehicles could be withdrawn because of safety fears follows 77 reported cases of braking problems among cars sold in Japan and 100 similar complaints in the US.

Prius owners have reported momentary loss of braking ability at low speeds on bumpy roads. Two of the incidents reportedly ended in crashes that resulted in injuries.

US safety regulators opened a formal investigation today.

The company is already reeling from a recall of 8.1m cars worldwide because of accelerator pedals that stick down causing vehicles to speed out of control.

Today Toyota executives in the UK said 250 owners had reported problems with accelerator pedals and Toyota had been able to confirm 20 cases, a far higher number than previously known.

The UK government withdrew from service a fleet of 10 Avensis cars used by junior ministers. Worldwide the accelerator problem is likely to cost the company £1.3bn in repairs and lost sales, it said.

Since its launch in 1997 the Prius – which uses a combination of a petrol engine and electric motor and is the world's best-selling hybrid car – has been held up by Toyota as a shining example of cutting-edge green technology. Global sales have reached 1.6m and the car has received endorsements from celebrities, including the actors Thandie Newton and Miley Cyrus.

A spokeswoman for Toyota confirmed that a Prius recall was an option in the US and Japan, but said no decision had yet been made. "Toyota Motor Group in Japan can confirm an investigation [into the brake problem] but no other action regarding a recall campaign has been taken," she said.

The Reuters news agency reported the Nikkei newspaper in Japan stating the recall would be announced in conjunction with Japan's transport ministry and the US department of transportation.

"We have no information that such a decision has been made," said a ­spokesman for Toyota in the US. "We are working with the national highway ­traffic safety administration on a preliminary evaluation of the issues raised in owner complaints."

There are thought to be no plans to extend any recall of the Prius to the UK, where the company said there had been no complaints about the affected third-generation model, which went on sale to British customers last summer.

Toyota said US and Japanese customers had reported "inconsistent brake feel when the brakes are slowly applied and the anti-lock system was activated while driving over potholes, bumps, or slippery road surfaces".

Toyota made a change to the software in the braking control on the current Prius in late January.

"We can assure Prius owners [in the UK] that no safety issue has been ­identified with the braking system fitted to their cars, and that no problems have been reported by customers in the UK or Europe," said Miguel Fonseca, managing director of ­Toyota GB. "With safety as our main concern, Toyota will continue to investigate thoroughly any problems that are reported."

"Uncertainty over the Prius trouble will only prompt more consumers to dump Toyota. Since the Prius is among Toyota's top-selling vehicles, its quality troubles could certainly hurt hybrid sales and Toyota's overall earnings."